In recent years, attention has been focused on new power generating techniques excellent in energy efficiency or environmental friendliness, and water electrolytic cells and fuel cells have been developed as examples of electrochemical apparatuses using a solid polymer electrolyte as an ion conductor in place of an liquid electrolyte. Solid polymer type fuel cells using the solid polymer electrolyte are characterized by high energy density as well as easier start and stop than fuel cells of other systems due to their lower operating temperature. Therefore, they are under development as power supplies for electric motorcars, dispersed power generation and the like. Bonding resins for polymer electrolyte membranes or membrane/electrode assemblies used for the water electrolytic cells or fuel cells as mentioned above are required to have a high proton conductivity as a cation exchange membrane and an inhibitory activity of permeation of fuels such as hydrogen, and to be sufficiently stable chemically, thermally, electrochemically and mechanically. Therefore, perfluorocarbon sulfonic acid membranes to which have been introduced sulfonic acid groups, a typical example of which is “Nafion (registered trade mark)” manufactured by DuPont Inc., USA, have been mainly utilized as one which can be utilized for a long period of time. In the case where the perfluorocarbon sulfonic acid membrane is operated under the condition where the temperature is 100° C. or higher, however, the water content of the membrane rapidly decreases and the membrane is significantly softened as well. In addition, in a fuel cell wherein methanol is used as the fuel, reduction of performance in power generation occurs due to the permeation of methanol through the membrane, and thus the fuel cell cannot exert sufficient performance. Further, referring also to a fuel cell that is operated at a temperature around 80° C. using hydrogen as the fuel, an excessively high cost of the membrane is pointed out as an obstacle impeding the establishment of a fuel cell technology. The same problem is also pointed out in the case where perfluorocarbon sulfonic acid resins are used as the bonding resin. Furthermore, when fluorine-based ion exchange membranes such as a perfluorocarbon sulfonic acid-based ion exchange membrane are used as the polymer electrolyte membrane for fuel cells, problems such as contamination of harmful fluoric acid into exhaust gas depending on operating conditions, and infliction of large load to the environment at the time of wasting have also been risen.
In order to overcome such drawbacks, a variety of polymer electrolyte membranes wherein a sulfonic acid group has been introduced into a polymer that contains a non-fluorine-based aromatic ring have been studied. A polymer skeleton of polyarylene ether compounds, such as polyarylene ether ketones and polyarylene ether sulfones, is considered to be a promising structure, taking heat resistance and chemical stability into consideration, and sulfonated polyarylene ether sulfones (see, for example, Journal of Membrane Science (Netherlands) 1993, vol. 83, pp. 211-220 (Non-Patent Document 1)), sulfonated polyether ether ketones (see, for example, Japanese Patent Laying-Open No. 6-93114 (Patent Document 1)) and the like have been reported. These compounds are those obtained by reacting a polymer with a sulfonating agent so that a sulfonic acid group is introduced. Meanwhile, a method for directly obtaining a sulfonated polymer through polymerization using a sulfonated monomer has also been reported (see, for example, US Published Patent Application No. 2002/0091225 (Patent Document 2), WO 2003/095509 (Patent Document 3), WO 2004/033534 (Patent Document 4), Japanese Patent Laying-Open No. 2004-509224 (Patent Document 5), and Japanese Patent Laying-Open No. 2004-149779 (Patent Document 6)). For example, in Patent Documents 2 and 5, polymers obtained by introducing an ionic group such as sulfonic acid group into a heat-resistant polymer such as polyimide and polysulfone have been proposed as a hydrocarbon-based ion exchange membrane.
Even among these aromatic hydrocarbon-based membranes, however, there has been a demand for a polymer having a more excellent ion conductivity. In addition, when used as a bonding resin for fabricating an electrolyte membrane/electrode assembly, a polymer having a stronger joining property to a polymer electrolyte membrane has been demanded so as to improve its durability.
Generally, in the hydrocarbon-based ion exchange membranes, it is necessary to introduce more ionic groups so as to express an ion conductivity comparable to that of perfluorocarbon sulfonic acid-based ion exchange membrane. However, the more the amount of the ionic group is, the larger is the swellability by water, and this causes problems such as dimensional change and deterioration of physical characteristics at the time of moisture absorption. Accordingly, as disclosed in, for example, Patent Documents 4 and 6, hydrocarbon-based ion exchange membranes of which the swellability is more suppressed by improving the structure of polymers have been proposed.
If it is tried to make the swellability of a polymer smaller, however, there were sometimes cases where its physical durability was deteriorated. For example, its joining property to an electrode catalyst layer was deteriorated in the case of using it for polymer electrolyte membranes of fuel cells, whereby the problems, such as detachment between the polymer electrolyte membrane and the electrode catalyst layer in the polymer electrolyte membrane/electrode assembly and deterioration in durability, occurred sometimes.
Further, in a fuel cell using hydrogen as a fuel, decomposition of an ion exchange membrane occurs due to radicals produced by a side reaction. Since hydrocarbon-based ion exchange membranes are inferior to perfluorocarbon sulfonic acid-based ion exchange membranes in regard to radical-resistant properties, such properties are improved by the addition of a hindered amine-based compound, a hindered phenol-based compound, an organic phosphorus compound, an organic sulfur compound or the like as a radical scavenger (see, for example, Japanese Patent Laying-Open No. 2003-183526 (Patent Document 7), Japanese Patent Laying-Open No. 2003-201403 (Patent Document 8), Japanese Patent Laying-Open No. 2003-151346 (Patent Document 9), and Japanese Patent Laying-Open No. 2004-047396 (Patent Document 10)), or by the use of an ion exchange resin containing a phosphonic acid group as an ionic group (see, for example, Japanese Patent Laying-Open No. 2003-238678 (Patent Document 11), Japanese Patent Laying-Open No. 2003-282096 (Patent Document 12), and Japanese Patent Laying-Open No. 2004-175997 (Patent Document 13)).
However, since most of such radical scavengers have a low molecular weight, there were problems such as bleed out and elution from the ion-exchange membrane. Further, ion exchange membranes made of an ion exchange resin containing phosphonic acid therein as an ionic group had drawbacks such as low ion conductivity.
As mentioned above, the actual circumstances are such that a solid polymer electrolyte membrane which is able to attain both good ion conductivity and good durability at the time of being used for a fuel cell has not been obtained.    Patent Document 1: Japanese Patent Laying-Open No. 6-93114    Patent Document 2: US Published Patent Application No. 2002/0091225    Patent Document 3: WO 2003/095509    Patent Document 4: WO 2004/033534    Patent Document 5: Japanese Patent Laying-Open No. 2004-509224    Patent Document 6: Japanese Patent Laying-Open No. 2004-149779    Patent Document 7: Japanese Patent Laying-Open No. 2003-183526    Patent Document 8: Japanese Patent Laying-Open No. 2003-201043    Patent Document 9: Japanese Patent Laying-Open No. 2003-151346    Patent Document 10: Japanese Patent Laying-Open No. 2004-047396    Patent Document 11: Japanese Patent Laying-Open No. 2003-238678    Patent Document 12: Japanese Patent Laying-Open No. 2003-282096    Patent Document 13: Japanese Patent Laying-Open No. 2004-175997    Non-Patent Document 1: Journal of Membrane Science (Netherlands) 1993, vol. 83, pp. 211-220